1. Field of the Invention
The present invention relates generally to the field of high-voltage switches, reactors, and circuit-switching devices, and more particularly to an impedance arrangement that is useful to limit transients during both the closing and the opening of a circuit or that is useful as a tuning reactor or current-limiting reactor.
2. Related Art
When a circuit-switching device associated with back-to-back capacitor banks is closed, inrush currents may reach values of 10 to 30 thousand amperes and high-magnitude voltage transients are produced. On energizing single capacitor banks, the inrush currents are lower but voltage transients are still produced. Such transient currents and/or voltages can produce undesirable noise, both audible and electrical, and can also, of course, lead to distress or damage of items connected to the circuit. In particular applications and dependent on the circuit parameters, the frequencies of these transients are in the range of 200-750 hz. Additionally, transients may also be created during the deenergization of power systems including reactance elements. For example, high-voltage, high-frequency transients in the frequency range of 10 to 100 khz may occur when a circuit-switching device is opened. These transients can stress insulation and cause deterioration over time or disruptive discharges.
Examples of the transient conditions that occur in power systems are discussed in an article by Bayless, et al, entitled "Capacitor Switching and Transformer Transients," 1986 IEEE PES Summer Meeting, Paper No. 86SM 419-6. This article also discusses various methods to limit transients including switch-closing resistors, switch-opening resistors, controlled closing, capacitor-bank reactors, and surge arresters. Additional examples of various arrangements that utilize reactances to limit currents and/or voltages in high-voltage circuits are disclosed in the following U.S. Pat. Nos. 3,376,475; 3,614,530; 3,697,773; 3,836,819; 3,927,350; 4,405,965; 4,550,356; and 4,567,538. For example, U.S. Pat. Nos. 3,376,475, 3,836,819, 3,912,975, 3,927,350, 4,184,186, 4,550,356, and 4,567,538 are directed to various insertion or switching arrangements to limit fault currents. U.S. Pat. Nos. 3,614,530 and 3,697,773 are directed to limiting transients. The mutual inductance variations between movable windings is utilized in U.S. Pat. No. 4,405,965 to limit current. Current flow through the coils produces a force to increase the effective inductance and thereby limit the current. Both series and parallel arrangements are utilized in different embodiments.
A pre-insertion inductor arrangement is disclosed in copending U.S. application Ser. No. 890,425 filed on July 24, 1986 in the name of Raymond P. O'Leary. This arrangement is effective to limit transient inrush current and/or voltages during the closing of a circuit by a circuit-switching device. The pre-insertion inductor is in the circuit only briefly during closing of the circuit-switching device. Thus, the pre-insertion inductor is not required to carry system momentary or short-time currents and need only carry the current of the circuit during the portion of the insertion time after the inrush. Further, an effective impedance at the inrush frequencies is achieved with a pre-insertion inductor that is approximately the same size and lighter in weight than pre-insertion resistors. Since the pre-insertion inductor has relatively low losses, the energy dissipation requirements are significantly lower than for pre-insertion resistors. While the losses and the reactance of the pre-insertion inductor are low at the source frequency of the circuit, the effective impedance at the inrush frequencies is substantially higher since the inrush frequencies are typically 5 to 10 times higher than the source frequency.
Accordingly, the pre-insertion inductor arrangement is effective to limit transient-inrush currents and/or voltages incident to circuit closing when energizing capacitor banks. While the pre-insertion inductor arrangement is an improvement over prior arrangements regarding the limitation of transients during circuit closing, in certain situations the pre-insertion inductor can combine with other reactance in the circuit to result in high-frequency transient voltages during circuit opening. This can also occur with fixed reactors which are connected in the circuit or other prior-art combinations of opening inductors, etc.
Thus, in certain applications, the use of the pre-insertion inductor or a fixed reactor might be precluded due to the creation of high-frequency transients on circuit opening. A simple wire-wound damping resistor is not practical since the high-voltage wire-wound damping resistor also includes inductance that represents a high impedance at the high frequencies of the oscillating transient. Additionally, simple damping even utilizing a non-inductive resistor would not be extremely effective since the resistance of the damping resistor must be high enough to be capable of limiting energy dissipation during the pre-insertion time.